The present invention relates in general to a motor vehicle braking system and, more particularly, the invention relates to an electrically-controlled fluid pressure responsive motor vehicle braking system.
In an electrical brake system, especially for those used on utility vehicles, the brake cylinders are activated by a pressure medium, which is generally compressed air. But the brake pressure, in this application, is not controlled by means of a control pressure graduated by a pedal-operated brake valve. Instead, the brake pedal is equipped with an electrical command value selector, for example, a potentiometer. The output signal of the selector is transmitted to an electronic system. In the electronic control system, the signal can be modified by additional devices, which can operate, for example, as a function of the load (ALB - automatic load-controlled braking), as a function of the wheel skid (anti-locking brake system), and/or in harmony with supplemental brakes (retarders).
The resulting braking system is transmitted to solenoid valves (modulators) which control the feed of the pressure medium to the brake cylinders. Such a solenoid valve can be constructed according to various principles (digital or analog). In place of a simple control, the braking pressure can also be set to a desired value. In this case, the actual pressure is generally monitored and indicated by a pressure sensor.
Such electrical brake systems have the advantage of achieving a rapid and more precisely-controlled braking. In addition, the hysteresis of the pedal-operated brake valve and other control valves (automatic load-controlled brake, etc.) can be eliminated in such systems.
Naturally, there are high safety requirements for braking equipment. Thus, for example, international standards specify that even if a failure should occur in the electrical system, the electronic system or a modulator of an electronically-controlled service brake installation on a motor vehicle, a limited, so-called emergency or residual braking action must be capable of being achieved. This interational safety requirement necessitates at least a two-circuit design of the service brake installation.
As an alternative to a two-circuit electronic system and electrical voltage feed - by analogy to hydraulic back-up brake installations, like those which are common in the USA and which are known in the prior art-it is conceivable to provide a pneumatic redundance for a single-circuit electronic system and voltage supply. For this purpose, the brake pedal, in addition to the electrical command value selector, also activates an ordinary pressure modulation valve. By means of the modulated pressure, if there is a total failure of the electronic or electrical system, the brake cylinder can be controlled directly by means of multiway valves, as has been taught by the prior art, or indirectly by means of appropriate modulators, which has also been shown in the prior art. Even in case of a failure, the wheel brakes can thereby be activated purely pneumatically. A disadvantage of these solutions is that the pneumatic circuit, which is inactive in normal operation, must be monitored by at least one additional pressure sensor.